This invention relates generally to improvements in video recorder-playback machines, and more specifically, to an improved spindle clamp assembly for securely positioning and accurately centering a video information disc on a rotatable drive spindle of a video recorder-playback machine.
Video recorder-playback machines in general are known in the art and include appropriate means for recording and/or playing back a video information signal utilizing a selected medium for storage of the signal. For example, in one type of machine, the video signal is recorded magnetically for storage and/or retrieval upon a length of so-called video tape. In another type of system, the video signal is recorded upon an information disc for retrieval by means of a stylus, in a manner analogous to that of sound reproduction from phonographic records. In still another type of system, the video signal is used to frequency modulate a beam of amplified light, such as a laser beam, with sufficient power for physically altering the surface characteristics of a light sensitive coating on a video information disc and thereby record the signal upon the disc. During playback, a lower power light beam is reflected from the disc and the resulting signal is demodulated to reproduce the recorded signal. In all of these types of systems, the video information signal can be combined with an appropriate audio signal for either recording or playback purposes to yield a composite audio-video signal of the type commonly used for television transmissions and the like. For convenience, however, the signal will be referred to herein as a video information signal.
Video recorder-playback machines utilizing amplified light beams for recording and playback purposes offer significant advantages in that all physical contact of the recording and playback elements with the storage medium, namely, the information disc, is avoided. This prevents wear and deterioration of the machine elements and the disc, resulting in a high quality stored video signal which can be played back repeatably over a long period of time with unimpaired video resolution.
In video recorder-playback machines using amplified light beams for signal recording and playback, the recording and playback optical beams are focused upon the video information disc by a focusing lens which traverses the span of the disc in a radial direction simultaneously with rotation of the disc. In this manner, a spiral pattern of closely spaced tracks is formed on the disc to represent the video information. For maximum storage capacity for each disc, togehter with maximum signal resolution, the information tracks are formed to have a narrow width such as on the order of about 0.5 microns, and the center-to-center spacing between radially adjacent tracks in the spiraling pattern is on the order of about 1.5 microns. Accordingly, to prevent cross talk between the tracks during recording and/or playback operation, it is imperative that the video information disc be rotatably driven about a precisely located and repeatably obtained axis of rotation to minimize radial eccentricities. In the prior art, this accurate positioning and centering has been obtained by precisely forming an axially centered opening in the video information disc for reception of the disc upon a correspondingly precisely formed rotating spindle. However, with this construction, the disc opening and spindle tolerances are crucial for accurate centering of the disc within the allowable range of radial error. This centering problem becomes increasingly apparent with prolonged use since the central disc opening tends to enlarge over time as a result of wear.
Another problem encountered in the prior art is that the disc must be securely clamped upon the spindle for consistent and accurate rotation therewith without slippage. This slippage problem is compounded by the relatively high speed of normal disc rotation, such as about 1800 rpm, together with the fact that a plurality of information discs tend to display a variation in thickness whereby any clamping means for securing a disc in position must be adapted for accommodating discs of varying thickness. Moreover, in a mastering machine for recording information upon a master disc for use subsequently in making production disc replicas, the master disc is typically formed from a fragile material such as polished glass. The clamping means must be adapted for securing the disc upon the spindle without substantial strain forces to avoid risk of breakage during clamping or high speed rotation.
The present invention overcomes the problems and disadvantages of the prior art by providing an improved spindle clamp assembly for a video recorder-playback machine which accurately and repeatably centers a video information disc upon a rotatable drive spindle, and which securely clamps the disc in position upon the spindle without excessive strain forces.